The purpose of this proposal is to investigate the characteristics of the receptors for the C3d fragment of complement (CR2 Receptors), we have found on human thymocytes and discern their biologic significance. The presence of CR2 will be demonstrated at the protein level by immunoprecipitation with specific monoclonal antibodies and polyacrylamide gel electrophoresis, and at the RNA level by northern blotting techniques using specific oligonucleotides. In addition, some of the physicochemical characteristics of the thymocyte CR2 will be analyzed and compared to the CR2 of B cells. Two dimensional electrophoresis and tryptic peptide analysis will be performed. The ability of thymocyte CR2 to bind EBV and its ligand, C3d, will be assessed. The former will be accomplished through binding studies with fluorescein-conjugated or radiolabelled virus. The latter will be studied by binding with fluorospheres coated with C3d. The expression of CR2 will be correlated with the appearance of other thymocyte markers. In particular, those which characterize mature of immature thymocytes. The ability of the CR2 ligand, C3d, to induce functional responses in thymocytes will be studied. These will include proliferative responses, expression of activation markers such as IL-2 receptors, release of IL-2 or other Lymphokines, and induction of thymic differentiation. The proposed studies will elucidate the biologic significance of CR2 on thymocytes and its possible association to thymic differentiation and/or function. Furthermore, the data obtained may provide new insights in the significance of EBV binding to CR2.